Vitreous Enamel - Properties

Properties

Enamel may be transparent or opaque when fired; vitreous enamel can be applied to most metals. The great majority of modern industrial enamel is applied to steel in which carbon content is controlled to prevent reactions at the firing temperatures. Enamel can also be applied to copper, aluminium, stainless steel, cast iron or hot rolled steel, as well as gold and silver.

Vitreous enamel has many excellent properties: it is smooth, hard, chemically resistant, durable, scratch resistant (5-6 on the Mohs scale), long-lasting color fastness, easy-to-clean, and cannot burn. Enamel is glass, not paint, so it does not fade with UV light. Its disadvantages are its tendency to crack or shatter when the substrate is stressed or bent, but modern enamels are chip and impact resistant because of good thickness control and thermal expansions well-matched to the metal. Its durability has found it many applications: early 20th century and some modern advertising signs, interior oven walls, cooking pots, exterior walls of kitchen appliances, cast iron bathtubs, farm storage silos, and processing equipment such as chemical reactors and pharmaceutical process tanks. Structures such as filling stations, bus stations and Lustron Houses had walls, ceilings and structural elements made of enameled steel. One of the most widespread use of enamel is in the production of quality chalk-boards and marker-boards (typically called 'blackboards' or 'whiteboards') where the wear and chemical resistance of enamel ensure that 'ghosting' or unerasable marks will not occur, as with polymer boards. Since standard enameling steel is magnetically attractive, they may also be used as magnet boards. Some new developments in the last ten years include enamel/non-stick hybrid coatings, sol-gel functional top-coats for enamels, enamels with a metallic appearance, and new easy-to-clean technologies.

The key ingredient of vitreous enamel is a highly friable form of glass called frit. Frit is typically an alkali borosilicate chemistry with a thermal expansion and glass temperature suitable for coating steel. Raw materials are smelted together between 2,100 and 2,650 °F (1,149 and 1,454 °C) into a liquid glass that is directed out of the furnace and thermal shocked with either water or steel rollers into frit.

Color in enamel is obtained by the addition of various minerals, often metal oxides cobalt, praseodymium, iron, or neodymium. The latter creates delicate shades ranging from pure violet through wine-red and warm gray. Enamel can be transparent, opaque or opalescent (translucent), which is a variety that gains a milky opacity with longer firing. Different enamel colors cannot be mixed to make a new color, in the manner of paint. This produces tiny specks of both colors, although the eye can be tricked by grinding colors together to an extremely fine, flour-like powder.

There are three main types of frit. First, ground coats contain smelted-in transition metal oxides such as cobalt, nickel, copper, manganese, and iron that facilitate adhesion to steel. Second, clear and semi-opaque frits contain little coloring material for producing colors. Finally, titanium white cover coat frits are supersaturated with titanium dioxide which creates a bright white color during firing.

After smelting, the frit needs to be processed into one of the three main forms of enamel coating material. First, wet process enamel slip (or slurry) is a high solids loading product of grinding the frit with clay and other viscosity-controlling electrolytes. Second, ready-to-use (RTU) is a cake-mix form of the wet process slurry that is ground dry and can be reconstituted by mixing with water at high shear. Finally, electrostatic powder that can be applied as a powder coating is produced by milling frit with a trace level of proprietary additives. The frit may also be ground as a powder or into a paste for jewelry or silk-screening application.

• View into a glass lined chemical reactor

• Turb-mixer in a glass lined chemical reactor